Reactions of Atomic Hydrogen with Formic Acid and Carbon Monoxide in Solid Parahydrogen II: Deuterated Reaction Studies

Ultraviolet Photodissociation of Proteinogenic Amino Acids

The ultraviolet photochemistry of the amino acids glycine, leucine, proline, and serine in their neutral forms was investigated using parahydrogen matrix-isolation spectroscopy. Irradiation by 213 nm light destroys the chirality of all three chiral amino acids as a result of the α-carbonyl C-C bond cleavage and hydrocarboxyl (HOCO) radical production. The temporal behavior of the Fourier-transform infrared spectra revealed that HOCO radicals rapidly reach a steady state, which occurs predominantly due to photodissociation of HOCO into CO + OH or CO₂ + H. In glycine and leucine, the amine radicals generated by the α-carbonyl C-C bond cleavage rapidly undergo hydrogen elimination to yield methanimine and 3-methylbutane-1-imine, respectively. Breaking of the α-carbonyl C-C bond in proline appeared to yield 1-pyrroline, although due to its weak absorption it remains unconfirmed. In serine, additional products were formaldehyde and E/Z ethanimine. The present study shows that the direct production of HOCO previously observed in α-alanine generalizes to other amino acids of varying structure. It also revealed a tendency for amino acid photolysis to form imines rather than amine radicals. HOCO should be useful in the search for amino acids in interstellar space, particularly in combination with simple imine molecules.

Infrared Spectra of 1-Quinolinium (C9H7NH+) Cation and Quinolinyl Radicals (C9H7NH and 3-, 4-, 7-, and 8-HC9H7N) Isolated in Solid para-Hydrogen

Large protonated polycyclic aromatic hydrocarbons (H⁺PAH) and the corresponding nitrogen heterocycles (H⁺PANH) have been proposed as possible carriers of unidentified infrared (UIR) emission bands from galactic objects. The nitrogen atom in H⁺PANH is expected to induce a blue shift of the band associated with the CC-stretching mode of H⁺PAH near 6.3 μm so that their emission bands might agree better with the UIR band near 6.2 μm. We report the IR spectrum of protonated quinoline (1-quinolinium cation, C₉H₇NH⁺) and its neutral species (1-quinolinyl radical, C₉H₇NH) measured upon electron bombardment during the deposition of a mixture of quinoline (C₉H₇N) and para-hydrogen (p-H₂) at 3.2 K, indicating that the protonation and hydrogenation occur mainly at the N atom site. Additional experiments on the irradiation of C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by irradiation with IR light to generate H atoms via Cl + H₂ (v = 1), were performed to induce the reaction H + C₉H₇N. This method proved to be efficient for hydrogenation reactions in solid p-H₂; we identified, in addition to C₉H₇NH observed in electron-bombardment experiments, four radicals with hydrogenation at the C-atom site─3-, 4-, 7-, and 8-HC₉H₇N. Spectral assignments were achieved according to the behavior upon secondary photolysis and a comparison of experimental results with vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. The observed lines at 1641.4, 1598.4, and 1562.0 cm^-1 associated with the CC-stretching mode of C₉H₇NH⁺ are blue-shifted from those at 1618.7, 1580.8, 1556.7, and 1510.0 cm^-1 of the corresponding protonated naphthalene (C₁₀H₉⁺).

Hydrocarboxyl Radical as a Product of α-Alanine Ultraviolet Photolysis

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S₂ excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO₂ + H or CO + OH. On the other hand, no photoproducts were detected by S₁ excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N₂O, which induces a two-step tunneling reaction, H + N₂O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N₂O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N₂O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Diffusion of Water Molecules in Quantum Crystals

With the use of solid parahydrogen in matrix isolation spectroscopy becoming more commonplace over the past few decades, it is increasingly important to understand the behavior of molecules isolated in this solid. The mobility of molecules in solid parahydrogen can play an important role in the dynamics of the system. Water molecules embedded in solid parahydrogen as deposited were found to be mobile at 4.0 K on the time scale of a few days. The diffusion at this temperature must be due to quantum tunneling in solid parahydrogen. The diffusion dynamics were analyzed based on the theory of nucleation. The concentration dependence on the diffusion rate indicates that there might be correlated motion of water molecules, a signature of quantum diffusion. We find that both water monomers and water dimers migrate in solid parahydrogen and provide insight into the behavior of molecules embedded in this quantum crystal.

Signatures of a quantum diffusion limited hydrogen atom tunneling reaction

We are studying the details of hydrogen atom (H atom) quantum diffusion in parahydrogen quantum solids in an effort to better understand H atom transport and reactivity under these conditions.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices

The p-H₂ matrix-isolation technique coupled with photolysis in situ or electron bombardment produces protonated or hydrogenated species important in astrochemistry.

Quantum Diffusion-Controlled Chemistry: Reactions of Atomic Hydrogen with Nitric Oxide in Solid Parahydrogen

Our group has been working to develop parahydrogen (pH2) matrix isolation spectroscopy as a method to study low-temperature condensed-phase reactions of atomic hydrogen with various reaction partners. Guided by the well-defined studies of cold atom chemistry in rare-gas solids, the special properties of quantum hosts such as solid pH2 afford new opportunities to study the analogous chemical reactions under quantum diffusion conditions in hopes of discovering new types of chemical reaction mechanisms. In this study, we present Fourier transform infrared spectroscopic studies of the 193 nm photoinduced chemistry of nitric oxide (NO) isolated in solid pH2 over the 1.8 to 4.3 K temperature range. Upon short-term in situ irradiation the NO readily undergoes photolysis to yield HNO, NOH, NH, NH3, H2O, and H atoms. We map the postphotolysis reactions of mobile H atoms with NO and document first-order growth in HNO and NOH reaction products for up to 5 h after photolysis. We perform three experiments at 4.3 K and one at 1.8 K to permit the temperature dependence of the reaction kinetics to be quantified. We observe Arrhenius-type behavior with a pre-exponential factor of A = 0.036(2) min(-1) and Ea = 2.39(1) cm(-1). This is in sharp contrast to previous H atom reactions we have studied in solid pH2 that display definitively non-Arrhenius behavior. The contrasting temperature dependence measured for the H + NO reaction is likely related to the details of H atom quantum diffusion in solid pH2 and deserves further study.